152 Royal Society :— 
Table I.). “ Rich yellow” appears also as opposed to “ full blue” 
in the scale of complementary tints exhibited by mica in my “‘'Trea- 
tise on Light” (Encyc. Metrop., art.507). It is not asserted that 
either a good yellow or a good blue cannot be produced otherwise 
than in a particular manner, but that they eax be produced im that 
particular manner, and that when so produced, their union affects 
the eye with no sensation of greenness. 
Let two very narrow strips of white paper, A, B, be placed parallel 
to one another in sunshine, so as to be seen projected on a perfectly 
black ground (a hollow shadow), and viewed through a prism having 
the refracting edge parallel to them, the refraction being towards the 
eye, and let the nearer B be gradually removed towards A, so that 
the red portions of B’s spectrum shall fall upon the green portion of 
A’s. Their union will produce yellow, or, if too far advanced, 
orange. On the other hand, it will be seen that the yellow space in 
B’s spectrum on which the blue of A’s falls is replaced by a streak 
of white,—whiteness, and not greenness, being the resultant of the 
joint action of these rays on the retina. If the strips be made 
wedge-shaped, tapering to fine points, and A being still white, B be 
made of paper coloured with the yellow chromate of mercury before 
mentioned, the whiteness of the streak where the blue of A mixes 
with the yellow of B near the pointed extremities will be very 
striking. 
There is a certain shade of cobalt-blue glass which insulates, or 
very nearly so, a definite yellow ray from the rest of the spectrum, 
suppressing the orange and a great deal of the green. If the spec- 
trum of B, formed and coloured as last described, be viewed through 
this glass, a very well-defined image of it, clearly separated from its 
strong red and very faint blue images, will be seen. As the glass in 
question allows blue rays to pass, the white object, A, besides its 
definite yellow image, will form a broad blue, indigo, and violet train 
nearer to the eye. Now let B be gradually brought up towards A, 
so that the violet, indigo, and blue rays of this train shall coincide in 
succession with the yellow image of B,—n0 sensation of greenness will 
arise at any part of its movement. Again, if a white card be laid 
down on a black surface, the edge nearest the eye, when refracted 
towards the spectator by a prism, will of course be fringed with the 
more refravgible half of the spectrum. Let this be viewed through 
such a glass, and in the blue space so seen introduce one half of a 
narrow rectangular slip of paper thus coloured, having its upper edge 
in contact with the lower edge of the white card, the other half pro- 
jecting laterally beyond the card. In this arrangement the definite 
image of the yellow paper insulated by the glass will be seen divided 
into a yellow half, projecting beyond the blue fringe, and a purplish- 
or bluish-white one within it, hardly to be distinguished from the 
image of the white paper, of which it seems a continuation, and 
which through the glass in question appears a pale blue. This same 
purplish tint was observed to arise also under the following cireum- 
stances :—Laying down in a good diffused light a paper of an ex- 
ceedingly beautiful ultramarine blue, and beside it, and somewhat 
overlapping it, another coloured with the same yellow chromate, I 
